Modulation systems and methods are widely used in transmitters to modulate an information input including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators are also referred to as "mixers" or "multipliers". For example, in a mobile radiotelephone, a modulator is used for the radiotelephone transmitter.
In modern communications systems, it is often desired to provide dual-mode modulation systems and methods that can modulate two types of communications signals. For example, in mobile radiotelephones, it is often important to provide a modulator that operates both in narrowband FM mode and in wideband Code Division Multiple Access (CDMA) mode. More particularly, in order to provide a mobile radiotelephone that can be used with both an IS-19 AMPS analog system and an IS-95 Direct Sequence Spread Spectrum (DSSS) wideband CDMA system, it is desirable to provide dual-mode modulation systems and methods.
Unfortunately, it may be difficult to provide a dual-mode modulation systems and methods that can handle the disparate bandwidths of the AMPS and CDMA signals. In particular, the narrowband AMPS FM signal has a bandwidth of about 12.5 KHz, while the wideband CDMA signal has a bandwidth of about 615 KHz, or about an order of magnitude wider.
In modem radiotelephone communications, mobile radiotelephones continue to decrease in size, cost and power consumption. In order to satisfy these objectives, it is generally desirable to share circuitry in dual-mode radiotelephones. Shared circuitry can decrease the number of components that are used in the modulator, thereby allowing a decrease in the size thereof. Shared components can also decrease the power consumption of the dual-mode modulation system, which can allow an increase in battery time. Finally, sharing of components can allow a decrease in component cost, thereby allowing a decrease in the overall cost of the radiotelephone.
FIG. 1 illustrates a first conventional dual-mode modulator. As shown in FIG. 1, an IQ modulator 10, also referred to as a "quadraphase modulator" or a "quadrature modulator" includes a quadrature splitter 20, also known as a 90.degree. phase shifter, and a pair of multipliers 16a, 16b coupled to the quadrature splitter. A local oscillator 15, such as a Voltage Controlled Oscillator (VCO), is coupled to the quadrature splitter 20 to produce 90.degree. phased shifted local oscillator signals. I data 11a and Q data 11b are coupled to a respective multiplier or mixer 16a, 16b respectively. Digital input data is converted to analog data by I Digital-to-Analog Converter (DAC) 14a and Q DAC 14b, respectively. The outputs of the DACs 14a and 14b respectively are applied to low pass filters 12a and 12b respectively to provide the I and Q data inputs 11a and 11b respectively. The modulator modulates the input data on a carrier 13, by summing the outputs of the multipliers 16a, 16b at summing node 218, and transmits the modulated carrier 13 via an antenna.
The DACs 14a and 14b, low pass filters 12a and 12b and IQ modulator 10 may be used to modulate a high bandwidth CDMA signal such as a Direct Sequence Spread Spectrum (DSSS) signal onto a carrier. Since the signal is generated digitally, it is low pass filtered by filters 12a and 12b to let the information through while removing digitally generated spurs and noise.
In order to use the IQ modulator 10 of FIG. 1 in a dual-mode, such as for narrow bandwidth FM signal, a separate FM DAC 19 and a separate FM low pass filter 17 may be provided. Baseband circuitry generates an FM voltage signal that is applied to the tune line of the VCO, to modulate the FM information onto the carrier for transmission according to the AMPS standard. Since the FM voltage signal is generated digitally, it is low pass filtered by FM low pass filter 17 to let the information through while removing digitally generated spurs and noise.
The low pass filter 17 generally has a different bandpass characteristic than the low pass filters 12a and 12b that are part of the CDMA modulator, due to the widely differing bandwidths of the FM and CDMA signals. Accordingly, in this dual-mode embodiment, a separate FM DAC 19 and a separate FM low pass filter 17 is provided. Modulation systems according to FIG. 1 have been designed into many integrated circuit chip sets developed for CDMA standards that also include AMPS functionality. Unfortunately, this technique uses separate DACs and low pass filters, which may increase the size, cost and/or power consumption of the modulator.
A second dual-mode modulation system is illustrated in FIG. 2. In this figure, an IQ modulator 210 including a quadrature splitter 220, a pair of multipliers 216a and 216b, a summing node 218 and a VCO 215 are provided to produce a modulated carrier 213. However, in contrast with FIG. 1, the DACs and low pass filters are shared for the dual-mode operation. In particular, the I DAC and Q DAC 214a and 214b respectively are used for both wideband CDMA and narrowband FM operation. Low pass filters 212a and 212b are also used for wideband CDMA and narrowband FM operation.
Unfortunately, due to the widely disparate bandwidths of the CDMA signal and the FM signal, the low pass filters 212a and 212b should have different band pass characteristics when in the different modes. In order to share the low pass filter, the band pass frequency is switched depending upon mode. Accordingly, while these switched filters 212a, 212b are used in both modes, they may be expensive to implement and may consume excessive power and/or area in a radiotelephone.